Nearly 200 more people would die yearly from respiratory problems if all vehicles in the United States ran on a mostly ethanol fuel blend by 2020, the research concludes. The study's author acknowledges such a quick and monumental shift to plant-based fuels is next to impossible.

"If you want to use ethanol, fine, but don't do it based on health grounds. It's no better than gasoline, apparently slightly worse," said study author Mark Jacobson, a Stanford University civil and environmental engineering professor.

His study, based on a computer model, was published inWednesday's online edition of the peer-reviewed journal Environmental Science and Technology.

Farmers, politicians, industry leaders and environmentalists have clashed over just how much ethanol can be produced, how much land it would take to grow the crops to make it and how much it would cost. They also disagree on the benefits of ethanol in cutting back fuel consumption and in fighting pollution.

In January, U.S. President George W. Bush announced a push to reduce gas consumption by 20 per cent over 10 years by substituting alternative fuels, mainly ethanol. Scientists with the Environmental Protection Agency estimated that could mean about a one per cent increase in smog.

Environmentalists not persuaded

Jacobson's study troubles some environmentalists, including Roland Hwang of the Natural Resources Defense Council. Hwang said ethanol, which cuts one of the key ingredients of smog and produces fewer greenhouse gases, is an important part of reducing all kinds of air pollution.

"There's nothing in here that means we should throw away ethanol," he said.

Matt Hartwig, spokesman for the Renewable Fuels Association, the largest Washington ethanol lobby group, said other research and real-life data show "ethanol is a greener fuel than gasoline."

But Jacobson found that depends on where you live, with ethanol worsening the ozone problem in most urban areas.

Based on computer models of pollution and air flow, Jacobson predicted the increase in ozone — and diseases it causes — would be worst in areas where smog is already a serious problem: Los Angeles and the northeast U.S.

The science behind why ethanol might increase smog is complicated, but Jacobson said part of the explanation is ethanol produces more hydrocarbons than gasoline.Additionally, ozone is the product of hydrocarbons and nitrogen oxide cooking in the sun.